This invention relates to transformer coupled drive circuits, and in particular to such drive circuits in which the peak value of the voltage coupled by the transformer to the load remains substantially constant notwithstanding variations in the duty cycle of the drive waveform.
It is often desirable to couple a voltage to a load, while maintaining conductive isolation between the load and the source of the voltage. This may be accomplished by the use of a series blocking capacitor, or by the use of transformer coupling, both of which ordinarily provide the type of coupling known as alternating-current or AC coupling.
In transformer coupling with a series blocking capacitor of repetitive signals having a constant duty cycle, the signal appearing at the load side of the capacitor and transformer combination maintains a constant peak-to-peak value. When signals of variable duty cycle are coupled by this combination, the peak-to-peak value of the alternating voltage or current on the line or load side of the continues to be the same as the peak-to-peak value on the source side, but the peak magnitude changes on the load side in such a manner as to maintain an average value of zero volts. This change in peak value may be undesirable. The changes in peak value may be eliminated by circuits known as direct current (D.C.) restorers. Such D.C. restorers may include capacitors charged by rectified signal. It may take several cycles of operation before the D.C. restorer capacitor charges to the correct value, which may be undesirable for coupling short bursts of signal of variable duty cycle if a constant peak value must be maintained.
It may be desired to drive the gate of a power field-effect transistor (FET) of the metal-oxide-semiconductor type (MOSFET) with a signal. The gate-to-source capacitance of such a FET is substantial. If fast turn-on or turn-off transition times are required, the gate-to-source capacitance requires that the drive source have a low impedance, so that it can supply the relatively large currents required to quickly charge and discharge the gate-to-source capacitance.
The secondary winding of a transformer can provide the low drive impedance required for driving the gate of a power FET. In order to provide both fast charge and fast discharge, bidirectional drive must be provided. However, if a series blocking capacitor is not provided, and the duty cycle of the drive waveform is other than 50%, the average value of the primary and secondary current may be non-zero, and magnetic core of the transformer may not be reset, i.e., the core may saturate. In a saturated state of the core, the mutual inductance of the transformer windings decreases, and undesirably large currents may flow therethrough. Also, the voltage coupled to the load may decrease.